It looks to be cosmetic damage only. Notice that the bottom of the rub rail has drain holes in it. Maybe consider it just another drain hole, only larger.
With the boat in the water you probably won't even see it, based on the camera angle.
It should be a white plastic rub rail with black rubber insert. Rudy at D & R Marine,
https://www.drmarine.com
can probably set you up with a piece. Call, don't email. I don't know if he has a minimum length he sells or not. Maybe somebody will pop up with a left over piece.
Pull out the black rubber insert a ways, carefully cut out the damaged white plastic piece, insert new and put the rubber back.
Be careful if you try to epoxy a patch. You will be working on the vertical underside of the rub rail. Gravity will not be your friend. Any mistakes may turn into an ugly mess. Rub rail can be replaced if damaged. Once you epoxy a patch, it will be permanent.
O'day 28s are nice boats. Enjoy